Why Stress Makes Us Sick and the Mind-Body Connection
The notion that stress makes you sick have been around for a thousands of years. Even the ancient Greeks knew. It was just that with our modern medical technology and the fact that stress is so commonplace, we just forgot that. We now have pills and injections that offer a quick fix to the symptoms, but not necessarily to the underlying cause. We now know that that underlying cause by be contributed by our emotions and stress.
Of course, each person experiences stress differently and genetic difference gives different stress tolerances for different individuals. What may be stressful to one person, may not be stressful at all to another. So it is not the stressful event per se that causes us to be sick; but rather our own perception and response to that stress. This cumulative stress response can causes physiological changes which make us to be sick.
Science now know that there are physiological responses in the body that decreases the effect of the immune system.
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What is Stress?
Hans Selye coined the term "stress" (borrowed from physics) to be ...
"Stress is the state manifested by a specific syndrome which consists of all the nonspecifically-induced changes within a biologic system. Thus, stress has its own characteristic form and composition, but no particular cause. The elements of its form are the visible changes due to stress, which are addictive indicators expressing the sum of all the different adjustments that are going on in the body at any time."
Speaking less formally, he rephrased it as ...
"Stress is the nonspecific response of the body to any demand, whether is is caused by, or results in, pleasant or unpleasant conditions."
Mind Body Connection
The idea that our thoughts and emotions can affect our physical body is the idea of the mind body connection. For example, there have been reports of people who develop physical diseases shortly after emotional trauma.
The mind and body are intimately connected. It goes the other way too, the body can affect the mind. For example, if one is in chronic backpain due to a physical injury, it would affect our mood and ability to be happy.
Book: "The Balance Within"
Physiological Effects of Stress
Ester Sternberg MD says in an American Public Media radio program interview: "Change, novelty, is one of the most potent triggers of stress response." 
This is a good thing. In fight-or-flight situations as when you see a lion that is not supposed to be there, it is good that your stress response is triggered so that stress hormone cortisol is pumped out to make your heart beat faster and pump harder. This may just save you from being eaten.
However, when this trigger is chronically pumping out cortisol, it is then that these hormone decrease the ability of the immune system to fight disease. And we get sick as a result.
Stress Produces High Cortisol
The adrenal gland pumps out a steroid hormone called cortisol. Besides getting the heart ready to meet a stressful event, cortisol plays a role in the on/off switch of the immune system and in the regulation of the body's inflammation response.
In a paper by Ester Sternberg and Philip Gold, it writes ...
"cortisol is a potent immunoregulator and anti-inflammatory agent. It plays a crucial role in preventing the immune system from overreacting to injuries and damaging tissues. Furthermore, cortisol inhibits the release of CRH by the hypothalamus — which keeps this component of the stress response under control. Thus, CRH and cortisol directly link the body's brain-regulated stress response and its immune response." 
One role of the hormone cortisol in the body is to help regulate and dampen inflammation. In normal conditions, cortisol keeps inflammation in check. However, when the body that is under stress releases excessive cortisol hormone on a chronic basis, cortisol loses its effectiveness to do its job. Chronic exposure to cortisol down-regulates the cortisol receptors, making the cells less responsive to cortisol. It is like cortisol resistance.
In the book The Cortisol Connection: Why Stress Makes You Fat and Ruins Your Health, it writes ...
"...when stress is repeated or constant, cortisol levels go up and stay up, causing a third phase of the general adaptation syndrome that is often referred to as overload. In this overload stage, bodily systems start to break down and our risk for chronic disease skyrockets. This is when we begin to see problems associated with weight gain, immune-system suppression, depression, anxiety, lack of energy, and inability to concentrate." [page 32-33]
Stress and Autoimmune Diseases
High cortisol is damaging to the immune system and can lead to autoimmune diseases. In the book The Autoimmune Epidemic, it writes ...
"Prolonged levels of heightened cortisol can not only lead to underfunctioning immune reaction, but can also indirectly stimulate an autoimmune response." [page 250]
Chronic stress wears out the adrenal glands so that it is no longer able to produce enough cortisol, adrenaline, and norepinephrine. The book, The Immune System Recovery Plan explains ...
"it's a serious issue if you've got an autoimmune disease becasue this could have been the trigger that started your body's autoimmune response in the first place, .... Without enough cortisol to kill off the bad T cells, these cells can turn into the kind that attack your own body." [page 119 -120]
Stress lead to inflammation
And paper in the Proceedings of the National Academy of Sciences says exactly that in more scientific terms...
"These data provide support for a model suggesting that prolonged stressors result in GCR [glucocorticoid receptor resistance], which, in turn, interferes with appropriate regulation of inflammation."
Unregulated inflammation is the basis for many chronic diseases and it promotes the progression of existing diseases. For example, cardiovascular disease is partly due to inflammation of the blood vessels. Stress is associated with greater risk for infectious diseases, depression, and cardiovascular disease.
Chris Kresser talked the link between chronic stress and inflammation and explains cortisol resistance in his podcast.
ScienceDaily reports on study of 1000 patients which found greater lifetime traumatic events was associated with increased inflammation.
Sheldon Cohen of Carnegie Mellon University did a study that suggest that chronic psychological stress reduces the body's ability to regular inflammation responses. Carnegie Mellon news release reports Cohen explaining in this way ...
"When under stress, cells of the immune system are unable to respond to hormonal control, and consequently, produce levels of inflammation that promote disease. Because inflammation plays a role in many diseases such as cardiovascular, asthma and autoimmune disorders, this model suggests why stress impacts them as well."
Not only does high cortisol increases inflammation, high cortisol can cause high blood glucose and high cholesterol. Dr. Mark Hyman writes in Blood Sugar Solution that ...
"Chronically elevated cortisol causes increased blood sugar and cholesterol, depression, and even dementia, and promotes the accumulation of belly fat that we so commonly see in patients with insulin resistance or diabetes." [page 91]
Connection Between Stress and the Immune System
Technical paper "The Mind-Body Interaction in Disease" by Esther M. Sternberg and Philip W. Gold says ...
"there is growing evidence that a wide variety of such diseases are associated with impairment of the HPA axis and lower levels of CRH secretion, which ultimately results in a hyperactive immune system. ... The popular belief that stress exacerbates inflammatory illness and that relaxation or removal of stress ameliorates it may indeed have a basis in fact. The interactions of the stress and immune systems and the hormonal responses they have in common could explain how conscious attempts to tone down responsivity to stress could affect immune responses." 
There are also differences in physiological responses to physical stress versus emotional stress.
American developmental biologist Bruce Lipton writes in his book Spontaneous Evolution that ...
"When the body's central intelligence identifies threatening signals from the external environment, it activates a specialized system called the Hypothalamus-Pituitary-Adrenal (HPA) axis. Regulatory signals released by the HPA axis primarily include stress hormones, such as adrenaline and cortisol. These chemical signals constrict the blood vessels in the viscera, which preferentially redirects blood flow to the somatic system." [page 261]
This is effect is good for preparing for the "fight or flight" responses in the muscles and bones. However, when this effect is chronic, there will be less blood and energy supply to other essential organs.
Page 24 of UltraPrevention writes ...
"Stress ..., which plays a mjor role in disease and disability, and contributes to all of the leading causes of death, from heart disease to stroke to cancer."
There is growing evidence that stress can increase the risk of autoimmune diseases in which the immune system loses some of its regulation and attacks body own tissue.
The paper "Stress as a trigger of autoimmune disease" writes ...
"Physical and psychological stress has been implicated in the development of autoimmune disease, since numerous animal and human studies demonstrated the effect of sundry stressors on immune function."
In the book The Thyroid Solution, Ridha Arem writes ...
"In essence, stress will make your immune system lose the ability to distinguish between "self" and "not-self"...." [page 23]
In a competitive environment, it is understandable that one might want to push oneself to the max in order to get ahead or achieve some goals, etc. However, if one is not mindful and pushes oneself too far, that will cause stress. A series of late night working with a few hours less sleep in order to "accomplish more" and get one extra step forward may end up two extra steps back when one gets sick and have to miss a couple of days of work to recoup. So in the end, one can not really make the body go any further than its limits. By being mindful, we become better attuned to our body and know when it needs rests and when it has extra energy to push ahead.
Other articles that you might like
-  Audio from American Public Media: "Stress and the Balance Within"
The American experience of stress has spawned a multi-billion dollar self-help industry. Doctor Esther Sternberg, a biomedical researcher and neural-immune expert, is wary of that. But she says that until very recently, modern science did not have th
-  Hans Selye - Wikipedia
-  Book: "The Balance Within: The Science Connecting Health and Emotions": Esther M. Sternberg
-  The Nature of Stress
-  Stress and the Balance Within | "The Mind-Body Interaction in Disease"
-  American Autoimmune Related Diseases Association, Inc
Information about autoimmunity and autoimmune related diseases. The American Autoimmune Related Diseases Association. Take our autoimmunity survey